The Challenges Of Maintaining And Improving The Uncertainty
Of An Industrial Humidity Calibration Laboratory
Speaker Jack Herring, Michell Instruments, Inc., 11 Old Sugar Hollow Road, Danbury CT
06810, USA phone (203) 744-6881, fax (203) 791-2047, email jack.herring@michell.com
Author Andrew M. V. Stokes Michell Instruments Limited, Lancaster Way, Ely, CB6 3NW,
UK, phone +44 1353 658004, fax +44 1353 658095, e-mail andrew.stokes@michell.com
Abstract
The paper describes the issues associated with maintaining a busy industrial humidity calibration
laboratory and the challenges faced in trying to maintain the laboratory measurement uncertainty
whilst achieving a throughput of thousands of sensors per year.
The humidity calibration facility of Michell instruments is essentially split into two parts – a UKAS
(EA) accredited laboratory for high level measurements with direct traceability and audit path to NPL
and NIST standards, and a commercial laboratory providing lower level tertiary calibration of tens of
thousands of dewpoint sensors per annum. The UKAS section focuses on excellence rather than on
volume – hence the processes and procedures are largely manual and quite time-consuming, whilst
the commercial section is there to handle large volumes of sensors automatically and with the
minimum of human intervention. The two therefore require very different approaches in terms of
equipment, procedures and analysis and the uncertainty levels achieved reflect the type of operating
model used in each case.
Humidity calibration systems used by secondary laboratories have tended to be constructed either as
clones of National Standards, using a two-pressure or two-temperature generation method, or as
simple divided flow systems utilising calibrated vertical tube flow meters. The former are very
expensive to produce and have certain limitations in terms of usability. The latter are cheaper to
produce, but also suffer from inflexibility and difficulty in automation. Furthermore, these systems
tend to offer varying flow rates dependent on the generated humidity level. The humidity calibration
system described in this paper provides accurate and highly repeatable humidity generation using a
combination of liquid and gas mass flow controllers. It allows automated use through the integration
of a precision chilled mirror dew-point hygrometer that provides both the control feedback to the
generator and traceability to National Humidity Standards.
The paper describes the two processes, provides a practical consideration of the component
uncertainties and explores ways in which these uncertainties can be refined and minimised through
improved procedures, better equipment and careful operation. Also provided are detailed calculations
of the liquid and gas mass flow ratios used to derive appropriate humidity levels in the measurement
chamber. A novel technique to ensure sensitivity and stability of the generated humidity is described,
along with the techniques employed to ensure homogeneity of the humidified air. The paper also
describes the physical design and construction challenges that were overcome in producing a fully
integrated system. An uncertainty budget for the whole system is provided, indicating the key
contributory factors and suggesting ways in which the measurement uncertainty can be minimised.
2009 NCSL International Workshop and Symposium
1 Background
Calibration is the comparison of a measured value with the true value. It has a formal
definition under ISO standards:
“Calibration: the set of operations which establish, under specified conditions, the
relationship between values indicated by a measuring instrument….. And the corresponding
values of a quantity realised by a reference standard”
Adjustment is the setting of a minimum deviation from measured value and true value.
Again, according to ISO:
“Adjustment: the operation intended to bring a measuring instrument into a state of
performance and freedom from bias suitable for its use”
National Standard:
“A standard recognised by an official national decision to serve, in a country, as the basis
for fixing the value of all other standards of the quantity concerned.”
The national standard in a country is often a ‘primary standard’. Details of several prominent
national standards can be seen in figure 1, below.
Figure 1.
Prominent international standards laboratories.
Traceability:
“The property of the result of a measurement standard, generally international or national
standard, by an uninterrupted chain of comparisons”
All definitions taken from ISO 10 102-1: (1992)
2009 NCSL International Workshop and Symposium
Regular calibration of humidity
sensors is important not only to
maintain traceability, but also to
ensure the correct operation of
the process concerned. Most
humidity
instruments
need
calibration at least every year to
ensure correct performance some need calibration every few
months. Of course, calibration
can be taken to mean either
verification
or
adjustment,
followed by certification
Figure 2.
Calibration traceability hierarchy.
The term uncertainty is defined as the
tolerance band in which (at k=2) a
measured value will be in agreement with
the ACTUAL value for 95 % of the
measurements made. It is effectively a
confidence band for the measurement and
can be calculated by combining various
experimental and theoretical data on the
system and instruments in use, according
to well established formulae.
Figure 3.
Normal distribution chart.
The uncertainties of the instrument in use and the calibration reference are both important.
Whereas a traceable laboratory typically has an
uncertainty of the order of ±0.2°C dew point (k=2
coverage) a typical field instrument may have an
uncertainty ten times worse. The calibration of a
hygrometer will result in two components - an off-set
(often termed its accuracy) at a given point and an
uncertainty value. So for example, if it has an off-set
of +0.2°C dew point and the calibration uncertainty is
±0.1°C dew point, then you have 95 % confidence
that the actual value = (reading -0.2) ±0.1.
Michell Instruments Limited has been manufacturing and calibrating chilled mirror and
capacitive (impedance type) dew-point hygrometers for more than thirty years. During most
of that time, the basic method of calibration has been consistent. This process consists four
component parts:
2009 NCSL International Workshop and Symposium
o Preparation of a supply of clean, dry compressed air with a moisture content below
the minimum measurable range of the instrument or sensor to be calibrated
o Generation of a range of dew-point temperatures across the operating range of the
instrument or sensor to be tested, using a single- or multi-stage divided flow method
o Provision of a suitable test chamber to house the instruments or sensors under test
o Verification of the whole system performance on a continuous or sampling basis
using a traceable reference hygrometer
In the early years the performance
of these calibration systems was
limited by a number of factors, for
example the quality and stability of
the dry air source, the sensitivity
limitations of the mixing system
and the uncertainty of the reference
measurement instrumentation. In
more recent times individual
components of the calibration
system have been optimised to
produce a better overall uncertainty
for the particular calibration
process, or product type, for which
they have been designed.
Figure 4.
Typical calibration system overview.
Although based on the same physical principles, the often disparate needs of the various
types of calibration dictate very different forms. This is particularly true in the case of a
laboratory set up for UKAS calibrations and also set up for volume calibration of mass
produced hygrometers. The different challenges and their solutions, as implemented by
Michell Instruments, are discussed in the remainder of this paper.
2
UKAS Laboratory: Quality Over Quantity
Referring back to our definitions,
traceability is the property of the result of a
measurement
standard,
generally
international or national standard, by an
uninterrupted chain of comparisons. Clearly,
the longer the chain the more uncertainty is
introduced. The more uncertainty there is in
a measurement the greater the potential
error.
Figure 5. UKAS calibration hierarchy.
2009 NCSL International Workshop and Symposium
Therefore it follows that the aim of a secondary laboratory, such as one operating under the
UKAS accreditation scheme, is to provide calibrations with as short a chain of comparisons
back to the appropriate national standards as can be practically achieved. In the case of
Michell Instruments’ UKAS laboratory this means that the comparisons are performed
directly against our in-house reference standard chilled-mirror hygrometers. These in turn are
periodically calibrated against the primary national standards in the UK (NPL gravimetric
hygrometer) and USA (NIST). Michell first obtained UKAS (then NAMAS) accreditation in
1987 and have since developed and refined our SOP’s and established a large database of
calibration history on the various reference standard hygrometers, some of which have
calibration history right back to the mid 1980’s. The UKAS part of our facility now has XX
reference standards, Y traceable to NPL and Z to NIST. The laboratory has increased
capacity from its original one to one comparison to the level now where up to five working
standards or customer instruments can be compared to the reference standard simultaneously.
The process is however still largely a manual one and is labour intensive, and therefore
expensive.
Figure 6.
Michell Instruments’ UKAS accreditation.
The performance limitations of a calibration methodology are determined by a combination
of factors including:
•
Physical facilities of the laboratory used
• temperature control
• air quality
• stability of electrical power supply
•
Quality of staff
• training
2009 NCSL International Workshop and Symposium
•
•
•
•
attention to detail
repeatability
variations in staff
Calibration equipment
• Accuracy
• Repeatability
• Traceability
In any laboratory these factors can be quantified statistically and combined to give a measure
of that laboratory’s capability as an uncertainty budget. The methods for calculating an
uncertainty budget are well documented and beyond inclusion in a general paper of this type.
For illustration, the uncertainty budget at -90 °C dp is comprised of the following:
Components of Uncertainty Calculation at -90 °C dp
Parameter
Calibration of standard
Drift of standard
Repeatability
Bridge calibration
Bridge repeatability
Bridge drift
Contamination
Temp. gradients in condensate
Temp. gradients in mirror
Pressure difference
Mirror temperature fluctuations
Non linearity
Sampling uncertainties
Divisor
2
1.732051
1
2
1
1.732051
1.732051
1.732051
1.732051
1.732051
1
1.732051
1.732051
Uncertainty Standardised
0.41
0.205
0.5 0.288675135
0.01
0.01
0.014
0.007
0.005
0.005
0.0225 0.012990381
0.015 0.008660254
0.01 0.005773503
0.01 0.005773503
0.01 0.005773503
0.0319
0.0319
0.08 0.046188022
0.39 0.225166605
Standard Uncertainty
K=2
Figure 6.
0.42394224
0.84788449
Uncertainty budget calculation for -90°C dp.
The overall uncertainty of the UKAS section of the dew point calibration laboratory is shown
most clearly in the chart 1. On the following page.
2009 NCSL International Workshop and Symposium
Chart 1.
3
UKAS measurement uncertainty.
Tertiary Calibrations: Achieving High Volume While Maintaining Quality
Michell Instruments’ production output
exceeds 30,000 hygrometer sales per
annum, each of which requires a
traceable calibration.
Given the
limitations of the UKAS equipment
previously mentioned, this requires a
different approach to be taken. What is
required here are calibration systems
that can deal with multiple IUT’s
simultaneously with minimal additional
uncertainty and, preferably, minimal
operator intervention.
Figure 7. Tertiary calibration hierarchy
The issues associated with the design, validation and calculation of uncertainty for such a
system are detailed below.
A primary enabling step that allowed Michell Instruments to develop a new calibration
technique was their development of a range of sensors and instruments for which the
calibration process is much easier to automate. In the past low-cost sensors tended to have
used simple analogue trimming circuits to enable zero, span and linearity to be adjusted.
More modern transmitters and instruments have benefited from digital communications and
significant on-board memory capacity that in turn facilitate automated calibration processes.
2009 NCSL International Workshop and Symposium
A further factor in the need to improve reliability and automation of the calibration process is
the response speed of sensors and systems to changes in applied humidity. At the lower
humidity limit of the early systems (typically -75°C frost point, equivalent to 1 part per
million) the response time for a system to a dry-down from ambient conditions might have
typically been 24 to 48 hours. Current demands for instrumentation calibrated down to 100°C frost point or below (13 parts per billion) demands much longer dry down times due to
molecular absorption, even on carefully prepared and optimised electro-polished stainless
steel surfaces. It is common for 120 hour (or longer) dry down periods to be used in order to
ensure full system equilibrium prior to a calibration. These elongated calibration periods
increase the need for system reliability, not only to ensure proper calibration, but also to
ensure satisfactory throughput of product to meet customer demand. Typical stabilisation
times for a calibration system such as that described in this paper are shown below.
Dewpoint Temperature
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
+10
+20
Figure 8.
Minimum stabilisation time
5 days
12 hours
10 hours
8 hours
4 hours
2 hours
1 hour
1 hour
1 hour
1 hour
1 hour
1 hour
1 hour
Calibration system stabilisation times
For tertiary calibrations the objective was to produce a calibration system with the following
key criteria: overall uncertainty better than ±2°C frost point at the low end and better than
±1°C frost/dew point at the high end; greater set-point stability; fully automated; increased
capacity; self diagnostic; automatic sensor verification and self-adjusting.
3.1
Available Technologies: Pro’s & Con’s
3.1.1
Dry gas supply
Two alternatives were considered – dry compressed air and liquid nitrogen. Ultimately it was
decided that dry compressed air would be used, primarily on the grounds of safety, ease of
implementation and familiarity. Michell has always used dried compressed air and found the
method to be extremely reliable provided a sensible preventative maintenance programme is
in place for compressors and dryers. A two stage drying process of oil-free compressed air at
7 barg is used to give a dry air feed to the generator of around 13 ppb moisture content,
approximately -100°C frost point when expanded to atmospheric pressure. In theory a
nitrogen source, dried through a nitrogen cold trap, can generate much lower moisture levels,
but this was regarded as an un-necessary additional complication to the project. Practical
experience has shown that the dry air system, now implemented on eight calibration systems,
has resulted in zero down-time during more than two years of continuous operation.
2009 NCSL International Workshop and Symposium
3.1.2
Humidity Generator
A feasibility study was undertaken to consider three options for humidity generation: twotemperature generator, two pressure generator and mass flow mixing system. These three
technologies are amongst the most widely implemented within National Standards
laboratories and commercial calibration laboratories worldwide. A mass flow mixing system
was chosen because of the ability to produce a system with flow rate flexibility, easy control
and relatively low cost in a short time scale. The humidity generator is the main subject of
this paper and will be described in more detail in the following sections of this paper.
3.1.3
Test Chamber and Sampling System
Considerable applications experience and expertise were utilised in designing a sensor
calibration manifold and sampling system to ensure maximum integrity of the generated
humidity at the test point. The design of this part of the system is described in more detail
further on in the paper.
3.2
Calibration Integrity and Traceability
Although not strictly necessary, based on the nature of the humidity generation method, it
was decided to use a chilled mirror dew-point hygrometer as an on-line reference and as the
control element of the closed loop generation system. This hygrometer provides traceability
through an un-broken chain to NPL and NIST standards.
4
Generator Design
4.1
Basic Concept:
The basic design concept was to use a novel combination of cascaded air mass flow
controllers in conjunction with a proprietary liquid mass flow controller with controlled
vaporisation/mixing, in order to allow the generation of any humidity level between -100 and
+20°C frost/dew point, equivalent 13 ppb to 23,600 ppm. This represents a dynamic
concentration range of 2 x 106. As a result, extremely careful selection of mass flow
controllers was necessary to ensure an appropriate sensitivity and reproducibility of
generation.
4.2
Selection Of Suitable Mass Flow Controllers:
A complex mixing system comprising three stages of mixing of wet and dry air flows was
developed in conjunction with the Dutch manufacturer Bronkhorst Hi-Tec, using their model
EL-Flow liquid and gas mass flow controllers and a Bronkhorst CEM Controlled Evaporator
and Mixer unit. Figure 9 illustrates the basic working of the system.
2009 NCSL International Workshop and Symposium
Figure 9.
MFC based dewpoint generator schematic.
A pressurised pure water source is fed through the Liquid Mass Flow Controller, LFC, at a
controlled flow rate from 0 to 0.945 litres per minute. In the first stage of mixing this liquid
water is mixed with a source of dry air (at approximately -100°C frost point) controlled by air
mass flow controller GFC1 at a rate of between 0 and 5 litres per minute. This mixture then
passes through the CEM evaporator/mixing unit and into an air receiver that acts as a further
buffer, improving output stability of the
total system. A controlled amount of this
pre-mix air is then mixed in the second
stage using GFC2, with more dry air
through GFC3, the excess flow from the
first mixing stage being exhausted the
electronic pressure controller EPC1 and a
rotary vent valve. In a similar
arrangement, GFC4 and GFC 5 provide
third stage mixing of pre-mix (2) and dry
air, with the excess again vented through
EPC2. The resultant mixed gas is then
ready for delivery at a controlled flow
rate to the sensors under test.
Figure 10.
MFC based dewpoint generator.
2009 NCSL International Workshop and Symposium
Whilst relatively simple in principle, practical realisation of a good working system relies
both on the careful selection of the appropriate controllers and a sophisticated electronic
control system, which in the case of this generator system is developed as a Windows based
automated routine using C++ programming language. Table 1 on the following page lists the
six mass flow controller elements used in the generator with their operating flow range and
reproducibility.
Controller
Range
LFC
GFC1
GFC2
GFC3
GFC4
GFC5
0 to 12.5 g/h
0 to 10 Nl/min
0 to 2 Nl/min
0 to 10 Nl/min
0 to 0.2 Nl/min
0 to 10 Nl/min
Table 1.
Worst Accuracy,
Nl/min
0.125 g/h
0.1
0.02
0.1
0.02
0.1
Reproducibility,
Nl/min
<0.0125 g/h
<0.01
<0.002
<0.01
<0.002
<0.01
MFC selection.
The liquid mass flow controller is calibrated in grams per hour. This can be converted to a
volumetric flow rate of the evaporated water as follows:
1 gram mole occupies a volume of 22.4 litres at stp, so 18.001 g occupies 22.4 litres
Therefore a flow of 12.5 grams per hour is equivalent to 22.4 x12.5/18.001 = 15.555 litres per
hour. So, the Controlled Evaporation method is an effective method for precisely delivering a
saturation water vapour flow at a rate as low as 0.0026 litres per minute.
4.3
Calculation Of Theoretical Generator Settings:
Whilst the design of the generator allows for either open-loop or closed-loop operation, it has
been set up to operate in a semi-open loop configuration whereby the reference hygrometer is
used to validate the generated humidity and control the overall calibration cycle, but not to
provide direct closed loop control of the mass flow controllers. This latter feature can be
considered as a design modification that would further improve the system in the future.
In the open-loop configuration, mass flow controller settings are developed based on a simple
theoretical model using volumetric calculation of the liquid water and multi-stage dry air
mixing stages. A theoretical generated frost-point temperature is calculated from the mass
flow controller settings as follows, assuming dry gas feed is at 13 ppb moisture content. In
this example the target generated frost point is -90°C:
Stage 1:
Wet flow is 0.308 g/h water = 22.4*(0.308/18.001) = 0.3833 litres per hour
Dry flow is 5.001 l/min = 300.06 litres per hour at < 0.01 ppmV
Resultant flow is effectively 106*0.3833/300.06 = 1277.41 ppmV
(-17.64°C f.p.)
2009 NCSL International Workshop and Symposium
Stage 2:
Wet flow is 0.055 l/min @ 1277.41 = 0.055*1277.41/5.055 = 13.8986
Dry flow is 5.000 l/min @ 0.013 = 5.000*0.013/5.005 = 0.0130
Resultant flow is therefore (13.8986 + 0.0130) = 13.9116 ppmV
(-58.02°C f.p.)
Stage 3:
Wet flow is 0.041 l/min @ 13.9116 = 0.041*13.9116/5.041 = 0.1131
Dry flow is 5.000 l/min @ 0.013 = 5.000*0.013/5.005 = 0.0130
Resultant flow is therefore (0.1131 + 0.0130) = 0.1261 ppmV
(-88.46°C f.p.)
Three-stage mixing at even the lowest humidity levels provides a good repeatability, being
less than 0.2% of controlled value for each flow controller. Table 2 illustrates the relevant
flow settings for all six mass flowmeters required to generate a -90°C frost point.
Mixing Stage
Flow settings, Nl/min [g/h]
Stage 1
GFC1=5.001, [LFC=0.308]
Target frost point, Repeatability, °C
°C
-17.64
±0.02
Stage 2
GFC2=0.055, GFC3=5.000
-58.02
±0.01
Stage 3
GFC4=0.041, GFC5=5.000
-88.46
±0.01
Table 2.
Three stage mixing of a -90°C frost point.
This gives a system repeatability, excluding variations in dry air moisture content, of
±0.025°C frost point, calculated in quadrature based on maximum repeatability errors at each
mixing stage.
4.4
Inclusion Of A “Buffer” Air Reservoir:
Initial test results with the system indicated that a problem existed in achieving a stable
generated humidity. It was discovered that operation of the liquid flow meter at very low
flow rates, even with the addition of the Controlled Evaporator and Mixer, was giving rise to
an erratic evaporation rate associated with the evaporation of individual water droplets.
Whilst not expected to be a problem for the calibration of impedance type hygrometers, this
cyclic response with a peak-to-peak variation of approximately 0.5°C at +10°C dew point and
1°C at -20°C frost point was large enough to cause total system uncertainty levels to exceed
the required levels.
It was therefore decided to incorporate a secondary mixing chamber after the first stage premix in order to provide a time averaging of this fluctuation and reduce its peak-to-peak effect
from the above levels down to 0.04°C at +10°C dew point and 0.2°C at -20°C frost point.
2009 NCSL International Workshop and Symposium
The subsequent remixing ( x 2) of this first stage air mix improves the fluctuation further
down to levels that are insignificant compared to the overall system uncertainty.
Whilst selection of an appropriate air reservoir was important from the perspective of
pressure rating, for safety reasons, the materials of construction were less important as the
lowest pre-mix frost point at this stage is approximately -20oC, rendering adsorption/wall
effects insignificant.
Charts 2. and 3. show the effects of the air reservoir on the output stability at -20°C frost
point, plotted over a twenty minute period with no air dryer changeover, in order to de-couple
any variation that may be caused by imbalance between the efficiency of the two molecular
sieve desiccant columns.
Chart 2.
Generator output stability at -20°C frost point without air reservoir.
2009 NCSL International Workshop and Symposium
Chart 3.
4.5
Generator output stability at -20°C frost point with air reservoir.
Design Of The Sensor Calibration Chamber:
In the past, Michell had used multi-sensor
calibration chambers machined from a solid block
of stainless steel, with drilled and plugged gas
flow paths to minimise dead space and internal
volume. These blocks were expensive to produce
and had limited capacity due to the design,
accommodating a maximum of 24 traditional
sensors or 12 transmitters, due to the larger
housing size required for transmitter electronics.
For the new humidity calibration systems it was
decided to employ a new design of sensor
calibration chamber, to accommodate the larger
transmitter format and allow a much larger number
of sensors to be calibrated simultaneously. The
general arrangement of a typical calibration
manifold is shown in figure 11.
Figure 11.
A calibration manifold (Note the system is loaded from both sides).
2009 NCSL International Workshop and Symposium
A vertical format stainless steel manifold design was implemented, giving each of the eight
calibration chambers the capacity to house up to 250 transmitters during any calibration
cycle. The calibration manifold is welded construction with high integrity VCR coupling gas
inlet and outlet ports. Each transmitter is secured into the calibration tube using its sensor
guard thread, and is sealed to the manifold with an individual Teflon ‘o’ ring.
As a further enhancement, each calibration
chamber is mounted within a sealed
environment during the calibration process. The
cowling is purged with air at -100°C frost point
to minimise humidity gradients between the
inside and outside of the sensor calibration
chamber thus negating any leakage effects that
might result from a poor seal at the sensor ‘o’
ring. Tests have indicated that this can reduce
the differential (air out – air in) by more than
80%, so improving the calibration integrity of
the sensors under test and therefore reducing the
overall uncertainty of calibration.
Figure 12
View of a section of the laboratory
The air path from generator to sensor calibration chamber is maintained in electro-polished
10 mm OD stainless steel tubing with a minimum of bends and welded angle connections.
A 5 metre tail pipe after the calibration chamber, in 6mm OD stainless steel tube, is used to
reduce back diffusion effects.
5
Verification of System Performance
This calibration system is essentially a practical device for the calibration of capacitive type
dew-point sensors over the range -100 to +20°C dew point. As such, it is designed to give a
reasonable uncertainty in context of the nature of the sensors being calibrated. The intention
in design was to produce a highly reliable system that would allow an overall calibration
uncertainty for the finished product (capacitive sensors) of ±1°C dew point from +20 to
60 and ±2°C in the range -60 to -100°C frost point.
Table 3 on the following page illustrates the results of a series of 95 calibration runs, from
which the generation uncertainty and overall system uncertainty were calculated. It should be
noted that the overall combined system uncertainty incorporates the uncertainty of the
reference hygrometer, calibrated in Michell’s UKAS accredited laboratory over the range -75
to +20°C dew point. The reference hygrometer uncertainty below -75°C frost point is
determined from previous experimental data.
2009 NCSL International Workshop and Symposium
Target Dew Point, oC
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
10
18
Mean Actual Dew Point, oC -98.59 -89.80 -80.24 -71.30 -60.11 -50.25 -40.09 -29.81 -20.11 -10.05 0.14 10.40 18.08
Standard Deviation
1.13
0.74
0.48
0.67
0.25
0.23
0.23
0.17
0.19
0.14
0.25
0.17
0.16
Uncertainty (95% CI)
2.25
1.49
0.97
1.33
0.51
0.46
0.45
0.34
0.38
0.29
0.51
0.34
0.32
Max Value, C
-95.8 -87.8 -79.5 -70.1 -58.3 -49.7 -39.8 -29.4 -19.7 -9.7
0.8
10.8
18.6
Min Value, oC
-101.7 -91.8 -82.1 -73.7 -60.7 -51.2 -41.1 -30.6 -20.5 -10.4 -1.1
9.9
17.7
0.9
0.9
o
Spread, oC
5.9
4.0
2.6
3.6
2.4
1.5
1.3
1.2
0.8
0.7
1.9
UKAS Inst. Uncertainty
1.00 0.82 0.64 0.48 0.34 0.32 0.31 0.29 0.27 0.25 0.24 0.22
(95%CI), oC
Combined System
Uncertainty
2.46 1.70 1.16 1.41 0.61 0.56 0.54 0.44 0.47 0.38 0.56 0.40
(95% CI), oC
Table 3.
Summary of actual test results and calculation of combined system uncertainty.
As can be seen from the results, the target combined uncertainty from -60 to +20°C dew
point exceeds the design brief by an average of around 50%. Whilst the combined uncertainty
currently exceeds the ±2°C target at lowest frost point (not generated), it is expected that
significant improvements will result when the reference hygrometer can be calibrated against
a national standard to these levels and further enhancements are made to the system to
improve low end reproducibility.
6
Summary
A commercial calibration laboratory exists
to maintain a strong link with national
standards, and by appropriate means to be
able to offer calibration services with the
best possible overall uncertainty. The
practical challenge is to be able to meet the
requirements of low volume traceable
calibration and also to be able to provide
high volume calibrations with minimal
detriment to uncertainty. In this paper we
have seen how a high volume humidity
calibration system is able to provide
practical and reliable calibration of
capacitive dew-point sensors.
Figure 13.
2009 NCSL International Workshop and Symposium
Unloading calibrated transmitters
0.20
0.38
In conjunction with a fully automatic, computer control program, these systems are able to
generate a range of dew point levels, verify them in real time against a traceable reference
chilled mirror dew-point hygrometer, measure and record the data from the 250 sensors under
test and validate correct programming of sensor memory with the appropriate calibration
data. This allows the fully automatic calibration of capacitive dew-point sensors in a
controlled environment with no human supervision or involvement.
We have also demonstrated that these systems have, in the main, achieved their design brief
and have delivered a combined uncertainty of less than ±2°C dew/frost point across an
extremely wide measurement range of -100°C frost point to +20°C dew point. These systems
have been fully operational for several years now, each one capable of performing in the
region of 40 calibration runs per year, providing a total capacity of more than 80,000
calibrated dew-point sensors in a 12 month period. An extended validation programme has
demonstrated both the reliability and long-term stability of the system. So, in conclusion, we
have achieved our objective and have in one laboratory systems based on similar
technologies that are capable of low volume, high labour content, traceable calibration on the
one hand and high volume automated calibration on the other.
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2009 NCSL International Workshop and Symposium